The present invention relates to an optical modulator and more particularly to an acoustooptic modulator for changing the intensity of light beams by an optical diffraction effect of acoustic waves in an acoustooptic medium in response to electrical modulation signals.
The acoustooptic modulator has a number of advantages such as a low operating voltage, stability of operating characteristics against variation in ambient temperature, and high quenching ratio, compared with other high-speed optical modulators. The acoustooptic modulator is, therefore, used extensively in various recorders and display equipment which use laser beams.
The bandwidth of the modulation signals in which the acoustooptic modulator can be used efficiently is not very wide, about 20 to 30 ns minimum in rise time and fall time of diffracted light. The following are understood to be the reasons for restriction of the modulation speed. When the incident light is a Gaussian beam of diameter 2Wo (Wo is a radius at which the optical power density is 1/e.sup.2 of that at the center), the rise time t of the modulated light can be given by minimum t.apprxeq.1.3 Wo/V, where V is the velocity of the acoustic wave in the accoustooptic medium. The modulation speed can be increased by making the beam radius Wo smaller. When the beam Wo is made smaller, the spreading angle of the Gaussian beam increases. The spreading angle can be expressed by .lambda./(n. .pi.. Wo), where .lambda. is the wavelength of light in a vacuum, and n the refractive index of the medium. Unless Bragg conditions are met along the entire region of angle components of the incident light in the acoustoopic medium, only optical wave components of the angle matching these conditions will be diffracted. Thus, the diffraction efficiency decreases when the incident light beams are narrowed in order to increase the modulation speed. Further, the diffracted light deviates from the Gaussian shape.
To prevent this, the spreading angle of the acoustic wave should be increased to at least the same level as that of the spreading angle of light. To accomplish this, the transducer for exciting acoustic waves in the medium has to be shortened in the light passing direction. Suppose, for example, a lead molybdate (PbMoO.sub.4) crystal, which is a well known acoustooptic crystal, is employed as the acoustooptic medium and the frequency of the acoustic wave is set at 200 MHz. To secure a modulation speed of below 10 ns rise time for the diffracted light in this case, the length of the transducer has to be shortened to below 2--3 mm. However, the diffraction efficiency is in proportion to the length of the transducer (i.e., working length), as is well known. For this reason, greater acoustic-wave drive power will be required to increase the intensity of the diffracted light. Reducing this working length of acoustic waves against the light is equivalent to reducing the thickness of the diffraction grid. For this reason, the diffraction phenomenon progresses from the Bragg reflection to the Raman-Nath diffraction. Therefore, the maximum diffraction efficiency that can be theoretically attained is decreased compared with a greater working length, even if the acoustic wave power is increased.
As one method of giving wide-angle components to the surfaces of acoustic waves without reducing the working length, a cylindrical acoustic wave is excited in the medium. An acoustooptic modulator with a relatively high speed can be realized by this method (c.f., "IEEE JOURNAL OF QUANTUM ELECTRONICS," Vol. QE-6, No. 1, January 1970, p.p. 15-24).
However, the surface of the acoustooptic medium must be cylindrically finished in order to install such a transducer. High-frequency transducers that can be installed on such a surface are limited to a piezoelectric thin-film transducer formed by an evaporation or sputtering method. It is very difficult to bond a regular piezoelectric crystal plate on a curved surface. For this reason, a piezoelectric high-coupling material such as a lithium niobate (LiNb03) crystal cannot be used, and therefore an increase in drive power is inevitable. In conclusion, it is difficult to simultaneously attain a high speed and high efficiency modulation with conventional acoustooptic modulators.